網頁知識：Real memory與Swap space
What is Swap Space?
Swap space in Linux is used when the amount of physical memory (RAM) is full. If the system needs more memory resources and the RAM is full, inactive pages in memory are moved to the swap space. While swap space can help machines with a small amount of RAM, it should not be considered a replacement for more RAM. Swap space is located on hard drives, which have a slower access time than physical memory.
Swap space can be a dedicated swap partition (recommended), a swap file, or a combination of swap partitions and swap files.
Swap should equal 2x physical RAM for up to 2 GB of physical RAM, and then an additional 1x physical RAM for any amount above 2 GB, but never less than 32 MB.
M = Amount of RAM in GB, and S = Amount of swap in GB, then If M < 2
S = M *2
S = M + 2
Using this formula, a system with 2 GB of physical RAM would have 4 GB of swap, while one with 3 GB of physical RAM would have 5 GB of swap. Creating a large swap space partition can be especially helpful if you plan to upgrade your RAM at a later time.
For systems with really large amounts of RAM (more than 32 GB) you can likely get away with a smaller swap partition (around 1x, or less, of physical RAM).
ImportantFile systems and LVM2 volumes assigned as swap space cannot be in use when being modified. For example, no system processes can be assigned the swap space, as well as no amount of swap should be allocated and used by the kernel. Use the free and cat /proc/swaps commands to verify how much and where swap is in use.
The best way to achieve swap space modifications is to boot your system in rescue mode, and then follow the instructions (for each scenario) in the remainder of this chapter. Refer to the Red Hat Enterprise Linux Installation Guide for instructions on booting into rescue mode. When prompted to mount the file system, select Skip.
swapoff -a && swapon -a
指令說明：如果已經使用了Swap，清空當下且buffers / cache還有空間，在執行swapoff -a操作時，會觸發Swap中的內容交換到真實記憶體中，數據不會遺失。